System and method for FFT window timing synchronization for an orthogonal frequency-division multiplexed data stream

ABSTRACT

A system and method for determining an FFT window location for reception of an OFDM signal received over a transmission channel. The OFDM signal includes a plurality of symbols each having a guard interval. The system includes a correlation module that determines a location of maximum correlation in a first symbol, an FFT module to perform an FFT on the OFDM signal based upon an initial FFT window location, and an adjustment module. The adjustment module determines a plurality of permissible echo location options based upon the initial FFT window location, selects a permissible echo location option that corresponds most closely to the location of maximum correlation, and adjusts the initial FFT window location based upon the selected permissible echo location option so that the adjusted FFT window location includes substantially all of a useful symbol length of the first symbol while a maximum number of echoes are included within the guard interval of the first symbol.

BACKGROUND

1. Field of Invention

Aspects of the present invention are directed to the processing of anOrthogonal Frequency-Division Multiplexed signal, and more particularlyto a system and method of synchronizing an FFT window on an OrthogonalFrequency-Division Multiplexed signal so that the FFT window includessubstantially all of the useful data portion of a symbol and anyappreciable echo energy lies within the guard interval of the symbol.

2. Discussion of Related Art

In Orthogonal Frequency-Division Multiplexing (OFDM) systems,information such as compressed audio and/or video data is carried via alarge number of individual carriers (i.e., sub-carriers) in a frequencymultiplex. The frequencies of the sub-carriers are selected so that themodulated data streams are orthogonal to each other, thereby eliminatingcross-talk issues. Although each carrier transports only a relativelysmall amount of information, high data rates may be achieved by using alarge number of carriers (e.g., 2048, 4096, 8192, respectively termed 2k, 4 k, and 8 k mode) multiplexed together. The individual carriers aremodulated (e.g., using phase-shift keying (PSK) techniques, or amplitudemodulation techniques, such as Quadrature Amplitude Modulation (QAM)),with each carrier having a fixed phase and amplitude for a certain timeduration, during which a small portion of the information is carried.That small portion of information is called a symbol, and the timeperiod for which it lasts is called the symbol duration. The modulationis then changed and the next symbol carries the next portion ofinformation. Examples of known OFDM systems include DVB-T (Digital VideoBroadcasting-Terrestrial) Standard systems, T-DAB (Terrestrial DigitalAudio Broadcasting) Standard systems, 3G and 4G mobile phone wirelessnetwork systems, as well as others.

In OFDM systems, modulation and demodulation are performed using theInverse Fast Fourier Transformation (IFFT) and the Fast FourierTransformation (FFT), respectively. The time duration of a symbol is theinverse of the carrier frequency spacing, thereby ensuring orthogonalitybetween the carriers. In addition to the data that is carried by an OFDMsignal, additional signals, termed ‘pilot signals’ (whose value andposition are defined in the applicable standard, and are thus known bythe receiver) are inserted into each block of data for measurement ofchannel conditions and also for synchronization.

In order to overcome inter-symbol interference, a portion of each symbol(e.g., the first portion or the last portion) is copied and appended tothe beginning or end of the symbol. For example, in DVB-T standardsystems, the last portion of the symbol is copied and appended to thebeginning of the symbol as a cyclic prefix. In OFDM systems, and as usedherein, that copied portion of the symbol is termed the “guard interval”and its duration (or length) is typically denoted A, the duration of theoriginal symbol (i.e., the “useful symbol duration”) is typicallydenoted T_(U), and the increased symbol duration is typically denotedT_(S), where T_(S)=T_(U)+Δ. Provided that most (or ideally all) echoenergy from a prior symbol falls within the guard interval, the symbolmay still be recovered.

In an OFDM receiver, the received OFDM signal is demodulated to basebandusing some type of quadrature amplitude demodulation or phase shiftkeying demodulation, the resultant baseband signals are then typicallylow-pass filtered, and the filtered baseband signals are then sampledand digitized using analog to digital converters (ADCs). After removalof the guard interval, the digitized signals are then provided to an FFTmodule and converted back to the frequency domain. Because of thepresence of the guard interval, a nearly infinite number of optionsexist as to where to place the FFT window to evaluate the symbol. Ingeneral, it is desired to place the FFT window on the useful part of thesymbol (T_(U)), and so that all or nearly all echo energy lies withinthe guard interval (Δ) of the symbol. One known process for determiningwhere to locate the FFT window is described in U.S. Pat. No. 6,459,744B1 (hereinafter the '744 patent), which is incorporated by referenceherein, and is now functionally described with respect to FIGS. 1, 2A,and 2B herein, and which correspond to FIGS. 7, 1, and 2 of the '744patent, respectively.

As described in the '744 patent, a received time domain OFDM signal x(t)is sampled at a sampling frequency H_(S) and converted into thefrequency space by means of an N-point FFT 72. The sampled signal isalso provided to a means 76 for measuring a correlation of the guardinterval of the sampled signal. The means for measuring the correlationof the guard interval 76 includes a correlator and summing accumulator761, that is provided with the sampled signal x(t) and with the samesampled signal x(t) delayed by the useful symbol length T_(U).

As described in the '744 patent, under ideal conditions where there isno noise, no multiple paths (i.e., no meaningful echo energy), and noco-channel interference, the correlation of the guard interval precedingthe useful part of a symbol and the end of the useful part of the symbolmay not only be used for a “rough” temporal synchronization, but mayalso be used for a fine temporal synchronization of the FFT windowplacement. This is illustrated in FIG. 1 of the '744 patent reproducedhere as FIG. 2A.

FIG. 2A illustrates a measurement of the correlation Γx(Tn), and thepulse response h(t) 13, for both a noise-affected idealized signal 11and for a noiseless idealized signal 12 that are received over atransmission channel with only one path 14 (i.e., having no meaningfulecho energy). As can be seen in FIG. 2A, where there is no meaningfulecho energy present, the correlation of the guard interval and the endof the useful part of the symbol will resemble a triangle with a definedpeak in the region of maximum correlation.

However, as also described in the '744 patent, where there issignificant echo energy present in the received signal, or where thereis a high level of interference, the correlation of the guard intervaland the end of the useful part of the symbol will be less well defined,and will resemble more of a deformed trapezoid, with each echo beingreflected by a correlation peak. For example, FIG. 2B illustrates ameasurement of the correlation Γx(Tn), and the pulse response h(t) 23,for both a noise-affected signal 21 and for a noiseless signal 22 thatare received over a transmission channel having two paths 24 ₁, 24 ₂spaced apart by a length of the guard interval Δ and received withidentical power (i.e., a main signal having an echo with the same poweras the original signal, and having a delay equal in length to the guardinterval). As can be seen in FIG. 2B, the main signal and the echo areeach reflected as a correlation peak, and although the correlation ofthe guard interval and the end of the useful part of the symbol maystill be used to determine a length of the OFDM symbol, its precisionmakes it difficult to discern where to optimally place the FFT window.

To overcome this deficiency, the '744 patent describes the use of ameans 77 for computing an estimation of the pulse response of thechannel. As described in the '744 patent, after the received signal x(t)is sampled and converted into the frequency space by means of theN-point FFT 72, those samples corresponding to one or more referencecarriers (i.e., pilot signals) are extracted and grouped to construct afictitious synchronization symbol in module 771. The fictitioussynchronization symbol is standardized by multiplication 772 and thensubjected to an inverse FFT 775 on N/R points (where N represents thenumber of orthogonal sub-carriers and R represents the spacing of areference sub-carrier every R sub-carriers) to provide an estimation ofthe pulse response (ĥ_(n)) of the channel.

The '744 patent describes that an analysis of the estimation of thepulse response of the channel may be used to determine the useful partof each symbol in the frame of the received OFDM signal, and to identifythe location of the main signal and any significant echoes. However, the'744 patent notes that under certain circumstances, an analysis of theestimation of the pulse response of the channel is incapable ofdistinguishing between a long echo (e.g., an echo having a delay greaterthan T_(U)/4 and less than T_(U)/3 of the current FFT window position)and a pre-echo. To remove this ambiguity, the '744 proposes the use ofthe correlation of the guard interval to remove the ambiguity inherentin the estimation of the pulse response of the channel.

As shown in FIG. 1, and as described with respect to FIGS. 6A and 6B ofthe '744 patent, the '744 patent provides a signal processing means 75that analyzes the spread of the correlation result provided by the meansfor measuring the guard interval correlation 76, and uses thatinformation to distinguish between a pre-echo and a long echo in theestimation of the pulse response of the channel. As described in the'744 patent, provided that the receiver was previously well synchronizedon the main path of the signal, the measurement of the correlation ofthe guard interval and the end of the useful part of the symbol isspread to a much greater extent in the case where the echo is a longecho than in the case of a pre-echo. Thus, by counting the number ofsamples that go beyond a given decision threshold, or by calculating theratio of the number of samples greater than this threshold relative tothe number of samples below the threshold, it is possible to distinguishbetween a pre-echo and long echo. Upon making a determination as towhether the echo is a long echo or a pre-echo, the signal processingmeans 75 uses that determination to adjust (i.e., delay or advance) thetiming of the FFT window.

As noted above, the '744 patent discloses how an analysis of theestimation of the pulse response of the transmission channel may be usedin conjunction with an analysis of the spread of the correlation of theguard interval and the end of the useful part of a symbol to adjust thelocation of the FFT window. However, the methodology used in the '744patent presumes that the FFT window was previously well synchronized onthe main path of the signal. In contrast, embodiments of the presentinvention are directed to systems and method for optimally locating anFFT window, irrespective of whether the FFT window was previously wellsynchronized.

SUMMARY OF INVENTION

Embodiments of the present invention are directed to a system and methodof determining an FFT window placement for the extraction of data froman OFDM signal, so that the FFT window includes substantially all of theuseful data portion of a symbol while all, or substantially all,appreciable echo energy lies within the guard interval of the symbol.Advantageously, embodiments of the present invention may be used toachieve a fast lock and high quality reception when a ratio of a lengthof the guard interval relative to the useful symbol length is greaterthan one half an effective pilot sub-carrier to total sub-carrier ratioof the OFDM signal, and irrespective of whether the FFT window waspreviously well synchronized. As used herein, the effective pilotsub-carrier to total sub-carrier ratio refers to the effective rate atwhich pilot signals are received in the OFDM signal.

In accordance with one aspect of the present invention, a method ofdetermining an FFT window location for reception of an OFDM signalreceived over a transmission channel is provided. The OFDM signalincludes a plurality of symbols each having a guard interval, and themethod comprises acts of a) determining, according to a first process, alocation of maximum correlation in a first symbol of the plurality ofsymbols; b) determining an initial location of the FFT window; c)determining a plurality of permissible echo location options based uponthe initial location of the FFT window and a second process, differentthan the first process; d) selecting a permissible echo location optionfrom the plurality of permissible echo location options that correspondsmost closely to the location of maximum correlation; and e) adjustingthe initial location of the FFT window so that the adjusted FFT windowlocation includes substantially all of a useful symbol length of thefirst symbol while a maximum number of echoes are included within theguard interval of the first symbol.

In one embodiment, the act (a) may include an act of determining,according to the first process, the location of maximum correlation inthe first symbol based upon a correlation between a first plurality ofsamples of the first symbol and a corresponding second plurality ofsamples of the first symbol that are spaced apart from the firstplurality of samples of the first symbol by the useful symbol length ofthe first symbol.

In another embodiment, the act (d) may include an act of comparing eachof the plurality of permissible echo location options determined in act(c) to the location of maximum correlation determined in act (a) toselect the permissible echo location option from the plurality ofpermissible echo location options that corresponds most closely to thelocation of maximum correlation. In accordance with this embodiment, andwhere the OFDM signal includes pilot signals, the act (c) may includeacts of extracting a plurality of pilot signals based upon the initiallocation of the FFT window; generating a channel estimate based upon theplurality of extracted pilot signals; and performing an Inverse FFT onthe channel estimate to generate a data structure identifying a locationand amplitude of each echo in the channel estimate. According to afurther aspect of this embodiment, the act (c) may further include actsof determining an initial echo location option based upon the locationand amplitude of each echo in the channel estimate, and determining theplurality of permissible echo location options based upon alternativepermissible locations for each echo in the initial echo location option.

Advantageously, embodiments of the present invention may be used todetermine the FFT window location for OFDM signals in which a ratio of alength of the guard interval relative to a useful symbol length of eachof the plurality of symbols is greater than one half an effective pilotsub-carrier to total sub-carrier ratio of the OFDM signal, and inaddition, where prior to the act of determining the initial location ofthe FFT window, the initial location of the FFT window was notsynchronized with the first symbol.

In accordance with another aspect of the present invention, a system fordetermining an FFT window location for extracting data in an OFDM signalreceived over a transmission channel is provided. The OFDM signalincludes a plurality of symbols, each having a guard interval, and thesystem comprises a correlation module to determine a location of maximumcorrelation in a first symbol of the plurality of symbols, an FFT moduleto perform an FFT on the OFDM signal based upon an initial FFT windowlocation, and an adjustment module, coupled to the FFT module. Theadjustment module determines a plurality of permissible echo locationoptions based upon the initial FFT window location, selects apermissible echo location option from the plurality of permissible echolocation options that corresponds most closely to the location ofmaximum correlation, and adjusts the initial FFT window location basedupon the selected permissible echo location option so that the adjustedFFT window location includes substantially all of a useful symbol lengthof the first symbol while a maximum number of echoes are included withinthe guard interval of the first symbol.

In accordance with one embodiment, the correlation module determines alocation of maximum correlation between a guard interval of the firstsymbol and a portion of the useful symbol length of the first symbol. Inone exemplary implementation, the correlation module calculates over afirst window that is the guard interval in length, a plurality of sums,each corresponding to a respective location of the first window as therespective location of the first window is moved along a useful lengthof the first symbol, of a correlation between time domain samples of thefirst symbol that are spaced apart from each other by the useful symbollength of the first symbol, for each of the time domain samples of thefirst symbol that are within the first window; calculates over a secondwindow that is the guard interval in length, a plurality of averages,each corresponding to a respective location of the second window as therespective location of the second window is moved along the usefullength of the first symbol, of the plurality of sums that are within thesecond window; and selects the respective location of the second windowhaving the highest average as the location of maximum correlation.

In accordance with another embodiment, the adjustment module includes apulse response estimation module, coupled to the FFT module, to performan estimation of a pulse response of the transmission channel, anddetermine an initial location and amplitude of each of a plurality ofechoes in the first symbol based upon estimation of the pulse responseof the transmission channel; and an echo options, analysis, andcomparison module, to generate the plurality of permissible echolocation options based upon the initial location and amplitude of eachof the plurality of echoes in the first symbol, compare a location ofeach echo in each of the plurality of permissible echo location optionsto the location of maximum correlation to select the permissible echolocation option from the plurality of permissible echo location optionsthat corresponds most closely to the location of maximum correlation,and to adjust the initial FFT window location based upon a location ofeach echo in the selected permissible echo location option so that theadjusted FFT window location includes substantially all of the usefulsymbol length of the first symbol while the maximum number of echoes areincluded within the guard interval of the first symbol.

In one embodiment, the pulse response estimation module may include apilots extraction and estimated channel module, coupled to the FFTmodule, to extract pilot signals from an output of the FFT module, andgenerate a channel estimate of the transmission channel; an Inverse FFTmodule, coupled to the pilots extraction and estimated channel module,to perform and Inverse FFT on the channel estimate, and generate achannel estimation spectrum that identifies each echo in the channelestimate; and an echoes list generation module, coupled to the InverseFFT module, to compare the amplitude of each echo in the channelestimate to a threshold, and identify the initial location and amplitudeof each of the plurality of echoes in the first symbol based upon eachecho in the channel estimate having an amplitude above the threshold.

In another embodiment, the echo options, analysis, and comparison modulegenerates the plurality of permissible echo location options based uponthe initial location and amplitude of each respective echo of theplurality of echoes in the first symbol and an alternative permissiblelocation for each respective echo.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings are not intended to be drawn to scale. In thedrawings, each identical or nearly identical component that isillustrated in various figures is represented by a like numeral. Forpurposes of clarity, not every component may be labeled in everydrawing. In the drawings:

FIG. 1 is a functional block diagram of a known device for synchronizingan FFT window placement according to the prior art;

FIG. 2A illustrates a measurement of the correlation of the guardinterval and the end of the useful part of a symbol and the pulseresponse of a transmission channel having no meaningful echo energy;

FIG. 2B illustrates a measurement of the correlation of the guardinterval and the end of the useful part of a symbol and the pulseresponse of a transmission channel having meaningful echo energy;

FIGS. 3A, 3B, and 3C illustrate how the results of a known pulseresponse estimation process can be ambiguous when the length of theguard interval of a symbol is greater than one half the length of theIFFT window over which that pulse response is determined;

FIG. 4 is a functional block diagram of an FFT window synchronizationdevice in accordance with an embodiment of the present invention;

FIG. 5 illustrates a channel estimate spectrum that may be provided by apulse response estimation module in accordance with the presentinvention;

FIG. 6 is an exemplary illustration of an echoes energy list that may beprovided by the pulse estimation module;

FIG. 7 graphically illustrates an initial echo location option that maybe determined by an Echo Options, Analysis, and Comparison module inaccordance with the present invention;

FIG. 8 graphical illustrates the manner in which the Echo Options,Analysis, and Comparison module determines permissible echo locationoptions based upon the initial echo location;

FIG. 9 is a flow chart illustrating steps which may be performed by theEcho Options, Analysis, and Comparison module to synchronize FFT windowplacement in accordance with the present invention; and

FIG. 10 is a functional block diagram of one exemplary implementation ofan FFT window synchronization module in accordance with an embodiment ofthe present invention.

DETAILED DESCRIPTION

Various embodiments and aspects thereof will now be described in moredetail with reference to the accompanying figures. It is to beappreciated that this invention is not limited in its application to thedetails of construction and the arrangement of components set forth inthe following description or illustrated in the drawings. The inventionis capable of other embodiments and of being practiced or of beingcarried out in various ways. Also, the phraseology and terminology usedherein is for the purpose of description and should not be regarded aslimiting. The use of “including,” “comprising,” “having,” “containing,”“involving,” and variations thereof herein, is meant to encompass theitems listed thereafter and equivalents thereof as well as additionalitems.

As used herein, the term “data” refers to physical signals that indicateor include information. The term “data” includes data existing in anyphysical form, and includes data that are transitory or are being storedor transmitted. For example, data may exist as electromagnetic or othertransmitted signals or as signals stored in electronic, magnetic, orother form.

A “memory” is a physical medium that can store data. Examples ofmemories include magnetic media such as diskettes, floppy disks, andtape; optical media such Magneto-Optic disks, CDs, and DVDs; andsemiconductor media such as semiconductor ROMs, RAMs, etc.

A display device refers to a device that can receive audio and/or videodata and provide a representation of that data in a human perceptibleform. Examples of display devices include screen display devices such astelevisions, computer monitors, PDA or cell phone screens, projectiondisplay devices, etc., as well as audio display devices such as radiosand other types of audio systems.

Prior to describing embodiments of Applicant's invention in detail, afurther discussion of the ambiguity that may be present when pulseresponse estimation is used to determine FFT window placement is nowdescribed with respect to a 2K mode (2048 carriers) DVB-T Standard OFDMsignal.

In the DVB-T Standard, the ratio of the length (duration) of the guardinterval (Δ) relative to the length (duration) of the useful symbol(T_(U)), termed the ‘Guard Length Ratio,’ may be 1/4, 1/8, 1/16, or1/32, meaning that the increased symbol length (T_(S)) is 1 part guardinterval to 4 parts useful data, 1 part guard interval to 8 parts usefuldata, etc. The sample time for N=2048 (2 k mode) is approximately 11micro seconds (11*10⁻⁶ seconds) per sample and for a Guard Length Ratioof 1/4, this corresponds to a guard interval of 512 samples. Because theGuard Length Ratio is an indication of the quality of the channel, thismeans that any appreciable echo energy should be confined to thisinterval, and thus it is possible to receive a first main echo, andanother, less strong echo 512 samples or 5.6 milliseconds (512samples×11*10⁻⁶ seconds per sample=5.6 milliseconds) away from the mainstrong echo. This is illustrated in FIG. 3A which depicts a first mainecho being received at a first time, and a second and weaker echo beingreceived at a second time 5.6 milliseconds later.

For many DVB-T Standard Systems, and as illustrated in FIG. 3 of the'744 patent, within a given symbol, the location of pilot signals arespaced apart from each other by 11 carriers containing useful data. Fromone symbol to the next symbol on an adjacent sub-carrier, the locationof the pilot signals is shifted (to the right) by three locations, suchthat the pattern of pilot signals is repeated every fourth symbol. Thus,when analyzed as a group of four or more symbols, the effective pilotsub-carrier to total sub-carrier ratio of the DVB-T Standard OFDM signalis 1 to 3, meaning that the pilot signals occupy 1 out of every 3frequency spots. If pulse response estimation is used to determine theposition of echoes, and the receiver extracts pilot signals every thirdfrequency spectrum bin, then a channel spectrum is obtained withfrequency bins 3 times larger than the sample frequency divided byN=2048. To estimate the channel in the time domain, an IFFT is performedon the channel spectrum. Because the frequency resolution (i.e., theminimal frequency bin) of the channel spectrum is 3 times larger thanthe sample frequency divided by N=2048, the IFFT can only see up to 1/3of the time that it took to collect the data, or 7.5 milliseconds((N/3)*(sample time)=(2048/3)*11*10⁻⁶=7.5 milliseconds). Thus, asillustrated in FIG. 3B, the viewing limit of the IFFT is +/−3.75milliseconds, and although the first main echo is located within theviewing limit of the IFFT, the weaker echo is located outside theviewing limit of the IFFT. Because of the cyclic property of both theFFT and the IFFT, after performing the IFFT, the echoes would appear asshown in FIG. 3C within the receiver, and the receiver cannot discernwhether the second and weaker echo came 1.9 milliseconds before the mainecho or 5.6 milliseconds after the main echo, since both possibilitiesare permissible for a guard mode (Guard Length Ratio) of 1/4 (i.e., bothoptions are within the 5.6 millisecond guard length interval).

It should be appreciated that the results of pulse response estimationare ambiguous under only certain conditions, and are not ambiguous underothers. For example, if in the example above, the effective pilotcarrier spacing was one every two sub-carriers rather than one everythree sub-carriers, then the frequency resolution of the channelspectrum would be only 2 times larger than the sample frequency dividedby N=2048. Accordingly, the IFFT would be capable of seeing up to 1/2the time that it took to collect the data, or 11.3 milliseconds (i.e.,((N/2)*(sample time)=(2048/2)*11*10⁻⁶=11.3 milliseconds), and thus, theviewing limit of the IFFT would be +/−5.6 milliseconds. Because allappreciable echo energy should be contained within the guard interval of5.6 milliseconds, there would be no ambiguity. Similarly, if instead theeffective pilot carrier spacing was maintained at one every threesub-carriers, but a 1/8 Guard Length Ratio (1/8 guard mode) were used,then the viewing limit of the IFFT would remain at +/−3.75 milliseconds,but any appreciable echo energy would be confined to a guard interval of2.8 milliseconds, and there would again be no ambiguity. It should beappreciated from the above that the results of pulse response estimationwill be ambiguous whenever the Guard Length Ratio is greater than onehalf the effective pilot sub-carrier to total sub-carrier ratio; thatis, whenever the Guard Length Ratio>(1/2)*(1/the effective rate ofreception of pilot signals in the OFDM signal, or alternatively, theeffective shift in location of pilot signals between adjacentsub-carriers of the OFDM signal). With respect to OFDM Standard systems,such as DVB-T Standard systems that specify an average effective pilotsub-carrier to total sub-carrier ratio of 3, ambiguity in the pulseresponse estimation will only be an issue whenever a Guard Length Ratiogreater than 1/6 is used (i.e., whenever the 1/4 guard mode is used, asthe 1/4 guard mode is the only DVB-T Standard guard mode having a GuardLength Ratio greater than 1/6), which ultimately represents a largeclass of real world systems.

In accordance with embodiments of the present invention, an FFT windowsynchronization module is provided that can achieve a fast lock and goodreception even when conventional pulse response estimation methods wouldprovide an ambiguous result, that is, whenever the Guard Length Ratio isgreater than 1/2 times the effective pilot sub-carrier to totalsub-carrier ratio of the OFDM signal. Advantageously, for DVB-T Standardsystems, embodiments of the present invention are therefore able toovercome echo ambiguity whenever a Guard Length Ratio of greater than1/6 is used (i.e., whenever a 1/4 guard mode is used).

FIG. 4 is a functional block diagram of an FFT window synchronizationmodule for optimally locating a FFT window on an OFDM signal so that anyappreciable echo energy lies within the guard length interval of asymbol in accordance with an embodiment of the invention. As depicted inFIG. 4, the FFT window synchronization module 400 is adapted to receivedigitized samples of the OFDM signal from an Analog to Digital (A/D)converter 410, and includes a Guard Interval Correlation module 420, aGuard Interval Removal and FFT Window Collection module 430, an FFTmodule 440, a Pulse Response Estimation Module 450, and an Echo Options,Analysis, and Comparison Module 460. As described further in detailbelow, based upon an analysis and comparison of the pulse estimationresponse and the guard interval correlation, the Echo Options, Analysis,and Comparison Module 460 provides a correction to the current FFTwindow placement so that the adjusted FFT window location includessubstantially all of a useful symbol length of the first symbol whilesubstantially all appreciable echo energy is contained within the guardinterval of a symbol.

As illustrated in FIG. 4, a baseband signal is digitized by the A/Dconverter 410 and the digitized samples are provided to both the GuardInterval Removal and FFT Window Collection module 430 and to the GuardInterval Correlation module 420. The Guard Interval Correlation module420 is capable of determining a relatively coarse FFT window position bycorrelating the digitized samples of the received OFDM signal, and thus,is frequently termed a Symbol Time Coarse Acquisition (STCA) module inthe art. As noted in the '744 patent, such a Guard Interval Correlationmodule may be implemented by comparing the sampled signal to a copy ofthe sampled signal delayed by the useful length (T_(U)) of the sampledsignal. However, and as noted in the '744 patent, where there issignificant echo energy and/or a high level of interference, such acorrelation between the guard interval and the end of the useful part ofthe sampled signal may not be sufficient to achieve a fine temporalsynchronization.

In operation, the Guard Interval Correlation module 420 determines acorrelation between samples of the signal taken a symbol length (i.e.,useful symbol length T_(U)) apart, and provides a metric, such as acorrelation index, identifying the location of maximum correlation. Forexample, in one embodiment, as a first step of the guard intervalcorrelation process, a sum, termed a ‘moving guard length sum,’ of thecorrelation between sample (n) and sample (n+symbol length (T_(U))) iscalculated for each sample that is within a window that is a guardinterval in length. This sum is continuously re-calculated as the windowover which the sum is calculated (and which is a guard interval inlength), is slid or ‘moved’ (for example, by one sample) along thesampled data.

Where the transmitted signal is received with little or no noise orinterference and any echo energy present in the received signal isminimal, the correlation provided by the first step of the guardinterval correlation process will have a maximum value where the windowover which the correlation is determined corresponds to the guardinterval. Such correlation is illustrated in FIG. 2A, which aspreviously noted, depicts a measurement of the correlation of the guardinterval and the end of the useful part of the symbol, and the pulseresponse h(t), for both a noise-affected idealized signal 11 and for anoiseless idealized signal 12 that are received over a transmissionchannel with only one path (i.e., having no appreciable echo energy). Ascan be seen in FIG. 2A, where there is no appreciable echo energypresent, the correlation provided by the first step of the guardinterval correlation process will resemble a triangle with a definedpeak in the region of maximum correlation.

However, and as noted previously, where there is significant echo energypresent in the received signal, the correlation provided by the firststep of the guard interval correlation process will be less welldefined, and will resemble more of a deformed trapezoid, with each echobeing reflected by a correlation peak. This is illustrated in FIG. 2B,which as previously noted, depicts a measurement of the correlation, andthe pulse response h(t), for both a noise-affected signal 21 and for anoiseless signal 22 that are received over a transmission channel havingtwo paths 24 ₁, 24 ₂ spaced apart by a length of the guard interval andreceived with identical power (i.e., having an echo with the same poweras the original signal, and a delay equal in length to the guardinterval). As can be seen in FIG. 2B, as a result of the first step ofthe guard interval correlation process, the main signal and each echoare reflected as a correlation peak, and it becomes difficult to discernwhere to optimally place the FFT window from this correlation alone.

To better identify where to place the FFT window, embodiments of thepresent invention utilize a second step of correlation, wherein a guardlength moving average is performed on the results of the first step.Thus, in the second step of the guard interval correlation process, anaverage of the moving guard length sums determined in the first step aretaken over another window that is a guard interval in length, as thatother window is slid or ‘moved’ (for example, by one sample) along thelength of the increased symbol duration (Ts). The second step of theguard interval correlation process effectively determines a guardinterval length's region of maximum correlation from the results of thefirst step, and provides a high variance but unambiguous index for theFFT window position. That is, the second step of the correlation processclearly indicates that the guard interval falls within the identifiedregion, although, the precise location at which it begins and endsgenerally cannot be identified from this metric alone.

For example, returning to FIG. 2B, and as a result of the first step ofthe guard interval correlation process, there are two local regions ofmaximum correlation, one identified at 0, and another identified at 0+Δ.As a result of the second step of the guard interval correlationprocess, the region of maximum correlation would be between 0 and +Δinclusive. In one embodiment of the present invention, the region ofmaximum correlation determined by the Guard Interval Correlation module420 may be provided as a correlation index (i.e., a data structure) thatis provided to the Echo Options, Analysis and Comparison module 460. Itshould be appreciated that this index may also be provided to the GuardRemoval and FFT Window Collection module 430 (indicated in dotted lineform in FIG. 4) to enable the Guard Removal and FFT Window Collectionmodule 430 to more quickly or better locate the guard interval forremoval.

As depicted in FIG. 4, and in addition to being provided to the GuardInterval Correlation module 420, the digitized samples of the OFDMsignal provided by the A/D converter 410 are also provided to the GuardRemoval and FFT Window Collection module 430. Using conventional guardinterval removal and FFT window collection techniques or utilizing thecorrelation index, the Guard Removal and FFT Window Collection module430 locates and removes the guard interval and attempts to place the FFTwindow on the useful length of the symbol T_(U). After removal of theguard interval, the FFT module 440 then performs an N-point FFT on thesampled signal, with the FFT result being provided to a Pulse ResponseEstimation module 450.

In broad overview, the Pulse Response Estimation module 450 extractsreference carriers (i.e., pilot signals) from the FFT result andgenerates a listing of the location of any appreciable echoes. In oneembodiment, the Pulse Response Estimation module includes a PilotExtraction and Estimated Channel Module 452, an IFFT module 454, and anEchoes List Generation Module 456.

The Pilot Extraction and Estimated Channel module 452 extracts andcollects pilot signals (i.e., reference signals transmitted with thedata that are used for frame, frequency, and time synchronization,channel estimation, etc.) from the OFDM symbol FFT result and passesthese signals through a low pass filter in time to generate a channelestimate (i.e., an estimate of the amplitude and phase shift of thereceived signal caused by the channel, that is based upon the pilotinformation). In the IFFT module 454, an IFFT is performed on thechannel estimate. The channel estimation spectrum consists of one ormore spikes, wherein the location of each spike corresponds to aparticular echo's location within the IFFT window, and wherein theamplitude of the spike corresponds to the energy of the echo, asdepicted in FIG. 5. It should be appreciated that the IFFT of thechannel estimate is equal to the Channel Impulse Response (CIR) in thetime domain. The process of determining echo location by use of anEstimated Channel IFFT result is frequently termed ‘ECIF’ in the art.

The channel estimation spectrum is provided to an Echoes List Generationmodule 456 which generates a data structure, for example a list, of eachecho location and the echo's power, for each echo having an energy(i.e., power) that is above a particular fixed or adaptive threshold.For example, in one embodiment, an adaptive threshold is used in whichonly echoes having a power that is not less than approximately 21 dBbelow the most powerful echo in the received signal is used. Thus, forexample, as depicted in FIG. 5, there are M distinct echoes, althoughonly the echoes at locations L₁, L₂, L_(M-1), and L_(M) have an energythat is above the threshold. An example of an echoes energy list isdepicted in FIG. 6, which graphically illustrates a listing of the echolocation and power of those echoes from FIG. 5 having a power that isabove the threshold.

The echoes list or data structure generated by the Echoes ListGeneration module 456 is provided as an input to the Echo Options,Analysis, and Comparison module 460. In broad overview, and asillustrated with respect to FIGS. 7-9, the Echo Options, Analysis, andComparison module 460 creates a plurality of possible echo locationoptions, based upon the provided echoes list, that identifies both thepower and location of each possible echo location. These possible echolocation options are then compared to the correlation index provided bythe Guard Interval Correlation module 420, and the possible echoeslocation option that is closest to the correlation index is selected asthat option corresponding to the true echo locations.

In accordance with one embodiment of the present invention, the EchoOptions, Analysis, and Comparison module 460 may perform a number ofdifferent steps or acts. For example, in a first step, the Echo Options,Analysis, and Comparison module 460 replaces each echo in the echoesenergy list with a triangle having have a base that is twice the guardinterval in length, and having a height that is equal to the echo'spower (i.e. amplitude), as graphically illustrated in FIG. 7. This stepof generating an initial echo location option based upon the echoesenergy list provided by the Echoes List Generation module 456 isillustrated in FIG. 9 as step 910. In a next step, depicted in FIG. 9 asstep 920, the Echo Options, Analysis, and Comparison module 460 performsa moving average over time, typically over an interval that is one tothree guard intervals in length, to produce an index or other type ofdata structure identifying the location and maximum value of each echo.In one embodiment, the interval over which the moving average isdetermined is one guard interval in length; although it should beappreciated the present invention is not so limited. For example, asillustrated in FIG. 8, Iteration 1, the moving average index wouldidentify a maximum value of P₁ at location Z₁=L (where L is the lengthof the guard interval), a maximum value of P₂ at location Z₂, a maximumvalue of P_(M-1) at location Z_(M-1), and a maximum value of P_(M) atlocation Z_(M).

The Echo Options, Analysis, and Comparison module 460 then moves (step930 in FIG. 9) the left most echo (e.g., the echo having the minimallocation within the window, in this case, the echo at location Z₁) to analternate permissible location that is spaced apart from the initiallocation by a distance of the useful symbol length multiplied by theeffective pilot sub-carrier to total sub-carrier ratio (i.e., at T_(U)/3away from its initial location, or at Z₁=L+T_(U)/3), and repeats themoving average process. Thus, for example, as illustrated in FIG. 8,Iteration 2, the left-most echo P₁ is moved to an alternate permissibleposition located a distance T_(U)/3 away, and the moving averagedetermination is repeated (step 920). As a result of the seconditeration, the moving average index would identify a maximum value of P₂at location Z₂, a maximum value of P_(M-1) at location Z_(M-1), amaximum value of P_(M) at location Z_(M), and a maximum value of P₁ atlocation L+T_(U)/3. This iterative process of determining the movingaverage over time and moving the next minimal location valued echo to analternate permissible echo location located a distance T_(U)/3 away isrepeated for each of the M echoes in the echoes energy list. Thisprocess of iteratively determining the location and maximum value ofeach possible echo option and identifying further options is reflectedin steps 920, 930, and 940 of FIG. 9.

After determining the maximum value and location of each echo for eachof the permissible echo location options, the Echo Options, Analysis,and Comparison module 460 compares each of the permissible options tothe current Guard Interval Correlation index, or to the average of a fewsymbols worth of correlation indices, and selects the option that is theclosest to the correlation index. The step of comparing each of the echolocation options to the current Guard Interval Correlation index isillustrated as step 950 in FIG. 9, and the step of selecting, as thetrue location of the echoes, that option which is closest to the GuardInterval correlation index, is illustrated as step 960.

From the selected option (step 960), the Echo Options, Analysis, andComparison module 460 then determines the true location of each echobased upon the selected option, and provides a window correction to theGuard Removal and FFT Window Collection module 430 that adjusts the FFTwindow location so that the adjusted FFT window location includessubstantially all of the useful data of a symbol while any appreciableechoes are within the guard interval of the symbol, so that receptiontiming is optimized in terms of the FFT window location placement. Thisstep of adjusting the FFT window location based upon the location of theechoes identified in step 960 is depicted as step 970 in FIG. 9.

In accordance with one embodiment of the present invention, and wherethe guard interval of a symbol includes a plurality of echoes having anappreciable amount of energy, the location of the FFT window is adjustedsuch that the FFT window opens after the start of the last appreciableecho contained in the guard interval of the symbol and prior to the endof the first appreciable echo contained in the guard interval of thesymbol.

In accordance with one embodiment of the present invention, the FFTwindow synchronization module 400 may be implemented using a combinationof dedicated and general purpose software and hardware to permit theadjustment of the FFT window location provided by the Echo Options,Analysis, and Comparison module 460 to be performed on each symbol ofthe OFDM signal. Advantageously, this permits embodiments of the presentinvention to achieve a fast lock and high quality reception even whensevere multi-path conditions are present, such as where the transmissionecho interval is greater than one half the effective pilot sub-carrierto total sub-carrier ratio of the OFDM signal.

For example, as depicted in FIG. 10, a functional block diagram of oneimplementation of an FFT window synchronization module 1000 is shownthat includes a processor module 1010, a memory module 1020, and ahardware assist module 1030, coupled together via a bus 1015. In apractical implementation, the window synchronization module 1000 may bepart of a larger system or device, such as a Set Top Box (STB), a HighDefinition (HD) Television, or radio, that receives digitized samples ofan OFDM signal (such as DTB-T or T-DAB) received from a tuner section ofthe device via satellite, from a wireless transmitter, or from aphysical transmission media, such as coaxial cable. It should beappreciated that the device or system in which the FFT synchronizationmodule 100 is implemented need not be a stationary device, but couldalternatively be a portable device such as a mobile phone, a laptopcomputer, or a PDA.

The processor 1010 can be some type of a programmable general purposeDigital Signal Processor (DSP), available from companies such as AnalogDevices, Motorola, or Texas Instruments, or an application specific DSPdesigned for a particular application and provided by a company such asZoran Corporation. The processor 1010 may be the same Digital SignalProcessor as that used for extracting the data from the OFDM signal andproviding the extracted data to a video and/or audio display device1040, such as a television display or a radio amplifier. As noted above,the display device 1040 may be either a standalone device (as shown), ormay be combined with the synchronization module 1000 into a integratedproduct.

The memory 1020 generally includes a combination of RAM memory and ROMmemory, but may also include other types of memory, such as flash ordisk-based memory, etc. In accordance with embodiments of the presentinvention, the memory may be adapted to store instructions for theprocessor 1010, as well as additional information, such as thecorrelation index, the initial echoes energies list, and the variousecho locations options as data structures within the memory 1020 foraccess by the processor 1010 and the hardware assist module 1030.

In one embodiment, the hardware assist module may be a Programmed orProgrammable Gate Array (PGA) encoded with instructions to perform thefunctionality of the Echo Options, Analysis, and comparison Module 460(FIG. 4) to enable adjustment of the FFT window position to bedetermined for each symbol. Based upon the adjusted FFW window position,the processor 1010 may provide digital video and/or audio data to thedisplay device 1040 for display to a user. It should be appreciated thatfor legacy systems still utilizing analog video and/or audio data, thedata could be transformed into the appropriate analog format using oneor more digital to analog converters prior to being provided to thedisplay device 1040.

It should be appreciated that although certain embodiments of thepresent invention are capable of adjusting the FFT window position foreach received symbol, the present invention is not so limited. In thisregard, the adjustment of the FFT window position could be determinedevery second or third received symbol, appreciating that changingchannel conditions (due, for example to the movement of the receiver,the transmitter, or both) may affect the quality of reception.Alternatively still, adjustment of the FFT window position could beperformed once every several blocks of the OFDM signal.

Having thus described several aspects of at least one embodiment of thisinvention, it is to be appreciated various alterations, modifications,and improvements will readily occur to those skilled in the art. Suchalterations, modifications, and improvements are intended to be part ofthis disclosure, and are intended to be within the scope of theinvention. Accordingly, the foregoing description and drawings are byway of example only.

1. A method of determining an FFT window location for reception of anOFDM signal received over a transmission channel, the OFDM signalincluding a plurality of symbols each having a guard interval, themethod comprising acts of: a) determining, according to a first process,a location of maximum correlation in a first symbol of the plurality ofsymbols; b) determining an initial location of the FFT window; c)determining a plurality of permissible echo location options based uponthe initial location of the FFT window and a second process, differentthan the first process; d) selecting a permissible echo location optionfrom the plurality of permissible echo location options that correspondsmost closely to the location of maximum correlation; and e) adjustingthe initial location of the FFT window so that the adjusted FFT windowlocation includes substantially all of a useful symbol length of thefirst symbol while a maximum number of echoes are included within theguard interval of the first symbol; wherein each of the plurality ofsymbols has a useful symbol length, and wherein a ratio of a length ofthe guard interval relative to a useful symbol length of each of theplurality of symbols is greater than one half an effective pilotsub-carrier to total sub-carrier ratio of the OFDM signal.
 2. The methodof claim 1, wherein the act (a) includes an act of: determining,according to the first process, the location of maximum correlation inthe first symbol based upon a correlation between a first plurality ofsamples of the first symbol and a corresponding second plurality ofsamples of the first symbol that are spaced apart from the firstplurality of samples of the first symbol by the useful symbol length ofthe first symbol.
 3. The method of claim 1, wherein the initial locationof the FFT window is based upon the location of maximum correlationdetermined in act (a).
 4. The method of claim 1, wherein the secondprocess determines the plurality of permissible echo location optionsbased upon the initial location of the FFT window and a pulse responseestimate of the transmission channel.
 5. The method of claim 1, whereinprior to the act of determining the initial location of the FFT window,the initial location of the FFT window was not synchronized with thefirst symbol.
 6. A system for determining an FFT window location forextracting data in an OFDM signal received over a transmission channel,the OFDM signal including a plurality of symbols each having a guardinterval, the system comprising: a correlation module to determine alocation of maximum correlation in a first symbol of the plurality ofsymbols; an FFT module to perform an FFT on the OFDM signal based uponan initial FFT window location; and an adjustment module, coupled to theFFT module, to determine a plurality of permissible echo locationoptions based upon the initial FFT window location, select a permissibleecho location option from the plurality of permissible echo locationoptions that corresponds most closely to the location of maximumcorrelation, and adjust the initial FFT window location based upon theselected permissible echo location option so that the adjusted FFTwindow location includes substantially all of a useful symbol length ofthe first symbol while a maximum number of echoes are included withinthe guard interval of the first symbol; wherein the correlation moduleand FFT module are implemented in a digital signal processor; andwherein each of the plurality of symbols has a useful symbol length, andwherein a ratio of a length of the guard interval relative to a usefulsymbol length of each of the plurality of symbols is greater than onehalf an effective pilot sub-carrier to total sub-carrier ratio of theOFDM signal.
 7. The system of claim 6, wherein the correlation moduledetermines a location of maximum correlation between a guard interval ofthe first symbol and a portion of the useful symbol length of the firstsymbol.